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X-RAY DEVICE AND DEPOSITION PROCESS
FOR MANUFACTURE
FIELD OF THE INVENTION
The present invention is directed generally to a method of manufacturing an X-ray emitter and, more particularly, to a method of forming an X-ray emitter having a diamond anode and a diamond housing.
BACKGROUND OF THE INVENTION
In the medical field, doctors and scientists are striving to find less invasive ways to treat patients. By using treatments that are less intrusive to the body, doctors can greatly reduce the stress on the patient's system and exposure to infection. For example, laparoscopic techniques enable physicians to explore the interior of the body and perform surgery through a small opening in the skin. Less intrusive medical techniques are extremely beneficial when applied to cardiovascular diseases, for example.
Cardiovascular diseases affect millions of people, frequently causing heart attacks and death. One common aspect of many cardiovascular diseases is stenosis, or the thickening of the artery or vein, which decreases blood flow through the vessels. Angioplasty procedures have been developed to reopen clogged arteries without resorting to a bypass operation. However, in a large percentage of cases, arteries become occluded again after an angioplasty procedure. This recurrent decrease of the inner diameter of the vessel is termed restenosis. Restenosis frequently requires a second angioplasty and eventual bypass surgery. Bypass surgery is very stressful on a patient, requiring the chest to be opened, and presents risks from infection, anesthesia, and heart failure. Effective methods of preventing or treating restenosis could benefit millions of people.
One treatment for restenosis that has been attempted is radiation of the vessel wall. For example, U.S. patent application Ser. No. 08/701,764, filed Aug. 22, 1996, titled "X-ray Catheter," describes an X-ray device for insertion into a lumen of a body, capable of localized X-ray radiation. U.S. application Ser. No. 08/701,764 is hereby incorporated by reference in its entirety. There are many difficult technical issues associated with delivering localized X-ray radiation to the interior of a patient's lumen. U.S. Pat. No. 5,854,822, titled "Miniature X-ray Device Having Cold Cathode" discusses improved cathode configurations that improve the rate of electron emission and decrease the required electric field. U.S. Pat. No. 5,854,822 is incorporated herein by reference in its entirety.
There is a need for effective devices to be used to treat the interior of the body with minimal intrusion. Effective, less invasive techniques for preventing and treating stenosis and restenosis at a lumen wall are especially needed. Size improvements on an X-ray device reduce the size of the required incision, improve maneuverability, decrease the stress on the lumen, and enable the device to reach more remote locations in the patient's body. Other applications for localized X-ray radiation are numerous, such as treating the interior of the esophagus, and providing radiation to tumors. Further, numerous non-medical applications require miniature x-ray devices that operate effectively, simplify manufacturing, and minimize the required voltage. For example, investigation of very small spaces can be performed using localized x-ray radiation.
SUMMARY OF THE INVENTION
Generally, the present invention relates to an x-ray emitter and a method for manufacturing an X-ray emitter. In one
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embodiment of the invention, a method of fabricating an X-ray emitter includes the steps of coupling a diamond housing to a diamond anode structure. The housing may include a diamond material that has a high resistivity while
5 the anode structure may comprise conductive diamond, in one alternative. The method may further include forming a target metal on the anode structure. In one embodiment, the target metal may have characteristic X-ray emission of at least 11 kiloelectron volts.
1° In another embodiment of the invention, a device for producing X-ray radiation includes a diamond housing, a cathode disposed within the housing, and a diamond anode structure, the anode structure coupled to the housing and the device arranged to enable the production of X-ray radiation.
15 The device may include a target metal on a tip of the anode structure. The anode structure may include graphite in one alternative embodiment. The housing may further include an external metallic layer in one embodiment. Alternatively, an exterior layer of the housing may include diamond doped
20 with boron to provide conductivity.
In yet another embodiment of the invention, a component for an X-ray emitter is described that includes a diamond housing coupled to a diamond anode structure.
25 The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiment
30 BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the 35 accompanying drawings, in which:
FIG. 1 shows a cross-sectional view of an X-ray device of the present invention.
FIG. 2 shows a side view of a primary mandrel. 4Q FIG. 3 shows a side view of conductive anode structure formed on a primary mandrel.
FIG. 4 shows a side view of the isolated anode structure.
FIG. 5 shows a cross-sectional side view of a secondary mandrel that covers portions of the anode structure. 45 FIG. 6 illustrates a cross-sectional view of a diamond housing formed on the anode structure and secondary mandrel.
FIG. 7 shows a cross-sectional view of the isolated anode-housing assembly, with a target metal formed on the 50 anode structure.
FIG. 8 shows a cross-sectional view of the anode housing assembly attached to an end cap cathode assembly.
FIG. 9 shows a typical X-ray spectrum composed of 55 Bremsstrahlung radiation and characteristic radiation.
FIG. 10 shows the relationship between the half value layer and energy for monoenergetic X-rays.
FIG. 11 shows the X-ray spectrum of a zirconium target. While the invention is amenable to various modifications 60 and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all 65 modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
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DETAILED DESCRIPTION OF THE VARIOUS
EMBODIMENTS
The present invention is believed to be applicable to a variety of devices, methods of fabrication, methods of use, systems and arrangements that irradiate X-ray radiation. The 5 invention is particularly advantageous for irradiating small, difficult to reach locations. For example, the present application is useful for irradiating lumens, vessels, or interior sites in a body using X-ray emitters to prevent restenosis in the cardiovascular system. While the present invention is not 10 so limited, an appreciation of various aspects of the invention will be gained through a discussion of the fabrication process and characteristics of such a device in connection with the examples provided below.
Generally, the present invention provides an improved 15 X-ray emitter, particularly an X-ray emitter that is designed for use inside a patient's body, especially a cardiovascular system. The method and device of the present invention produce a housing-to-anode connection that maintains a vacuum chamber despite temperatures changes. By using 2Q similar materials for both the housing and the anode base of the present invention, and by bonding the housing and anode base directly to each other, the x-ray emitter of the present invention is capable of maintaining mechanical integrity despite extreme temperature changes. The present invention 2J may also reduce the number of voids and spikes within an X-ray emitter that are capable of enhancing the electric field. Further, the present invention may result in a simplified manufacturing process.
The effect of localized x-ray radiation on living tissue will 30 now be discussed, to aid in understanding one application of the present invention. As X-ray radiation penetrates into the wall of the lumen or cavity, the radiation damages the DNA of a majority of smooth muscle cells. As the population of undamaged smooth muscle cells is depleted, their prolifera- 35 tion rate during the healing process after an angioplasty procedure is inhibited, and the consequent restentosis is less likely to occur. In coronary applications, it is desirable to have the X-ray radiation penetrate into the adventitia tissue of the blood-vessel about 1-2 millimeters deep from the 40 inner vessel wall. Penetration into the cardiac muscle tissue should probably be minimized, although differences of opinion exist within the medical field. It is further desirable to deliver X-ray radiation with a peak energy of about 8-12 kiloelectronvolts (keV) in coronary applications. When the 45 desired dosage has been delivered, the voltage source is discontinued and the X-ray device is withdrawn from the body.
X-ray emitters, particularly those that are miniature, require materials with particular specification requirements 50 for safe and effective operation within a body. Other application environments also require miniature X-ray emitters that operate without electrical or mechanical failure. Diamond, because of its mechanical, electrical and chemical properties, is useful in miniature x-ray emitters, meeting the 55 requirements for manufacturing the housing and the anode.
For example, for use in the body, the total diameter of the X-ray emitter should be small enough to readily pass through human arteries. The components of the X-ray emitter must be capable of construction at very small scales. 60 Preferably, the total diameter should be about 1-4 millimeters. Also, since a vacuum chamber is enclosed by the housing in the X-ray device, the housing material used should be capable of heat-resistant, vacuum-tight connections with the metal components and the anode and cathode. 65
Diamond structures meet these mechanical requirements. Diamond structures are mechanically stronger than the
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boron nitride structures previously used for X-ray devices. Constructing the vacuum housing and the anode with diamond permits a significant size reduction.
The housing material should have high X-ray transparency. The housing surrounds the anode and cathode components, where the X-ray radiation is produced. X-ray transparent housing material allows full and reproducible dosages. Diamond, because of its low atomic number is highly transparent to X-ray radiation, allowing all clinically significant X-rays to exit the housing.
The material for the X-ray device also requires particular electrical properties. At certain points in the X-ray device, the high-potential lead that is connected to the anode is separated from the low-potential lead that is connected to the cathode by a distance of less than a millimeter. High potential differences are present within an X-ray emitter across very small distances. Electrical current from the anode to the cathode along an inner wall or through the inner wall of the housing should be prevented. The housing material of the X-ray emitter should have a high dielectric strength, in order to withstand a large electrical field without breakdown.
High resistivity is a desirable quality for the housing material to prevent leakage current through the housing. Preferably the housing has a resistivity of at least lxlO11 ohm-cm. A bulk resistivity of lxlO13 ohm centimeters or higher is more preferable.
Other qualities of the emitter may also contribute to prevent electrical breakdown, such as the geometry of the emitter, lack of gases and contaminants in the vacuum housing, resistivity, surface resistivity, and the dielectric constant, as is known in the art. One X-ray device designed for use inside the body is described in U.S. patent application Ser. No. 08/701,764, filed Aug. 22, 1996, titled "X-ray Catheter," which is hereby incorporated herein by reference in its entirety.
Current that leaks through the housing does not generate X-rays, so an accurate X-ray dose may not be administered if current leakages occur. In addition, leakage current through the housing will also generate undesirable heat. Considerable amounts of heat can also be produced within the X-ray unit. The heat causes thermal expansion of the components of the X-ray emitter, and particularly with materials with significantly different thermal expansion coefficients, the heat may cause mechanical failure, such as cracking and distortion.
Diamond is also an excellent heat conductor, with a thermal conductivity of about 20 Watts/cm K. Therefore, the heat generated by the X-ray emitter, for example, as a result of electron bombardment on the anode, will be dissipated throughout the structure quickly. Mechanical failure, such as cracking and structural distortion of the emitter, can also be restricted by forming the housing and the anode from diamond since the components will have similar thermal expansion coefficients.
A further advantage of including diamond in the vacuum housing is the electrical resistivity of diamond. The electrical resistivity of chemically vapor-deposited diamond is approximately lxlO15 ohm-cm. The electric field at which diamond will experience electrical breakdown is about lxlO7 V/cm. In order to maintain an electric field at the surface of the cathode, the anode and high voltage carrying components of the X-ray unit must be insulated from the conductive coating and external conductive layer of the coaxial cable. The potential of the external conductive layer is a floating low potential. The patient is grounded, as is
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known in the art and as is described in the "Handbook of Electrical Hazards and Accidents," edited by Leslie Geddes, published by CRC Press, Boca Raton, Fla., 1995, which is hereby incorporated herein by reference in its entirety. Insufficient insulation results in electrical discharge or fail- 5 ure. The use of diamond as the vacuum housing improves insulation and reduces the likelihood of electrical failure.
FIGS. 1-8 illustrate one exemplary embodiment of a process for fabricating an X-ray emitter designed for use inside a patient's body, particularly for the cardiovascular 1° system. In particular, the housing and anode, both comprising diamond, are integrally coupled to reduce structural distortion due to heat exposure.
FIG. 1 illustrates a cross-section of one embodiment of an assembled x-ray device 100 of the present invention. Now 15 referring to FIGS. 2-8, the assembly steps of the x-ray device 100 are described. A conductive anode 115 is formed on a primary mandrel 110, as shown in FIGS. 2 and 3. The shape of the mandrel 110, in part, determines the shape of the anode 115, which is typically tubular and/or shaped so as 20 to form a tip 116. For example, the anode 115 may be a tapered cylinder with a rounded distal end, although many different shapes and other configurations for the anode 115 may be used and are contemplated by this invention. The mandrel 110 can be made of a variety of materials, for 25 example, silicon, tantalum, molybdenum, tungsten, titanium, or other appropriate materials which do not react with diamond and are easily removed after forming the anode 115. The primary mandrel 110 may be removed, for example, by etching with an acid, such as hydrofluoric acid. 30
The anode 115 comprises conductive diamond. The electrical resistivity of the conductive diamond anode typically ranges from, for example, 0.01 to 1x10s ohm-cm. The length of the anode 115 can range, for example, from 0.5 to 1.5 mm. 3J The thickness of the anode can range, for example, from 150 to 250 micrometers. Anodes of different sizes may be used, depending on the purpose of the device to be manufactured.
The anode 115 is typically formed by chemical vapor deposition (CVD) of diamond. Recent advances in chemical 40 vapor deposition techniques have made possible the construction of three-dimensional diamond structures. Diamond structures can be grown by depositing diamond onto a metal rod or mandrel 110.
The material for the diamond anode is electrically con- 45 ductive in order to establish the required electric field between the anode and the cathode. The conductive diamond anode 115 may be formed by doping the CVD plasma with for example, a boroncontaining compound, such as B2H2, or pure boron introduced into the deposition reactor. Atomic 50 dopant boron/carbon concentrations in plasma typically range, for example, from 50 to 500 ppm. Thus, in accordance with this invention, it is possible to use a threedimensional diamond shell as a structural element of the anode 115. 55
The most preferred methods of creating structural diamond parts are hot filament deposition, combustion, and direct current arc jets. These three types of chemical vapor deposition methods are described in the art and are generally known to those skilled in the art. For example, deposition of 60 diamond tube shapes is well-described in "Cylindrically Symmetric Diamond Parts by Hot-Filament CVD," Diamond and Related Materials, Volume 6, pages 1707-1715 (1997), written by T. R. Anthony, which is incorporated herein by reference in its entirety. Chemical vapor deposi- 65 tion of diamond is also described, for example, in the book Diamond Films and Coatings, Editor Robert F. Davis,
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Noyes Publication, 1993, which is incorporated herein by reference in its entirety. CVD of diamond can be performed by General Electric and other manufacturers.
After forming the anode 115 on the mandrel 110, the anode 115 is isolated, as shown in FIG. 4. Typically, the mandrel 110 is removed by etching the mandrel 110 and the anode 115 assembly in an acid solution, such as hydrofluoric acid. Other methods for removing the mandrel 110 may also be used, as long as the removal methods do not adversely affect the anode 115. The isolated anode 115 may then be cut, for example, by laser, to the desired size and may be cleaned, for example, using sulfo-chromic, nitric or sulfuric acid, to remove contaminants.
The anode 115 is then prepared to be coupled to the housing 125. A secondary mandrel 120 is positioned on the anode 115, as shown in FIG. 5. The secondary mandrel 120, including two pieces 120a and 120fc, is configured so as to selectively cover the anode 115, allowing the housing 125 to couple to the anode 115 and to define a vacuum chamber. Typically, the secondary mandrel parts 120a and 120fc are cylindrical members with center portions removed to accommodate the anode 115. The secondary mandrel 120 can be made of a variety of materials, for example, silicon, tantalum, molybdenum, tungsten, titanium, or other appropriate materials which do not react with diamond and are easily removed after forming the housing 125, for example, by etching with an acid.
The housing 125 is formed, as shown in FIG. 6. The housing 125 is coupled to a portion of the anode 115 and defines, in part, the shape of the X-ray vacuum chamber. The housing 125 typically has a cylindrical or tubular shape, such that it can be inserted into a patient's body to deliver X-ray radiation, although other configurations are possible and contemplated by this invention. The length of the housing 125 can range, for example, from 3 to 10 millimeters. The thickness of the housing walls 125 can range, for example, from 150 to 300 microns. Different sizes of housings may be used depending on the purposes of the device to be manufactured. The housing is made of insulating diamond with electrical resistivity typically higher than lxlO12 ohm-cm, for example.
The housing 125 is formed, typically, by chemical vapor deposition. Deposition of diamond is well-described in "Cylindrically Symmetric Diamond Parts by Hot-Filament CVD," Diamond and Related Materials, Volume 6, pages 1707-1715 (1997), written by T. R. Anthony, and in the book Diamond Films and Coatings, Editor Robert F. Davis, Noyes Publication, 1993, which were previously incorporated herein by reference in their entirety. CVD can be performed by General Electric and other manufacturers.
After the diamond housing 125 is formed, the housing 125 may be further treated. For example, the housing 125 may be annealed in air at a temperature of about 700° C. to 1000° C. for one-quarter to one hour in order to increase the electrical resistivity of the structure. The interior surface of the diamond housing 125 may also be treated in order to increase electrical resistivity of that surface. Etching of the inner surface with an acid, such as sulfo-chromic acid, increases the electrical resistivity and therefore helps reduce the risk of a short in the X-ray emitter due to a discharge between the high-potential anode and the cathode which is at a low potential. Heat treatment of diamond is described in M. I. Landstrass and K. V. Ravi, "Resistivity of Chemical Vapor Deposited Diamond Films," Applied Physics Letters, 55(10), Sep. 4, 1989, which is incorporated herein in its entirety.
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